The kidneys maintain constant levels of K+ in the plasma and extracellular fluids by matching urinary excretion of the ions to dietary intake. This process involves a complex signaling system through which the renal tubular cells interact with the plasma K+ concentration and perhaps also with the gut and endocrine organs. However the nature of this system is not well understood. We will examine how this process works by assessing the function of K+ channels in the apical membrane of the rat connecting tubule and collecting duct. These channels are thought to be the key pathway through which K+ is secreted into the urine. We will measure the overall activity of the channels using electrophysiological techniques, the total abundance of channels using immunoblotting, and the amount of protein in the apical membrane using a biotinylation approach for labeling proteins at the cell surface. We will correlate these parameters with parallel measurements of Na channel activity and NaCl-cotransporter expression. We will test the hypothesis that the serine/threonine kinase SGK1 plays an important role in the modulation of these processes and of renal K excretion. We will also assess the ability of insulin to acutely influence K+ transport in distal nephron segments. Finally, we will examine the interactions between these channels and intracellular Mg2+, changes of which may be important in pathological conditions involving Mg depletion or wasting.